1. Field of the Invention
This invention relates to techniques for inverting the polarity of an electric signal which is applied to liquid crystal so as to correspond to a plurality of frames or fields, when a liquid crystal display device is driven.
2. Description of the Related Art
In general, a thin-film-transistor (TFT) color liquid crystal display device includes a TFT substrate on which gate lines (scanning lines) and data lines (signal lines) are arranged, and another color filter substrate on which a common electrode is disposed. In this TFT liquid crystal display device, basically, a scanning signal is applied to the gate lines, while a corresponding display signal is applied to the source lines, so that matrix operation is performed, and pixels realize a high-quality image by performing a charge holding operation.
xe2x80x9cDriving by scanningxe2x80x9d means that a horizontal line of pixels is selected and a display signal is applied to the selected line. In driving a liquid crystal display device, the application of a direct current signal causes ions in liquid crystal to gather on one substrate, often resulting in deterioration of its liquid crystal display. To prevent this phenomenon, the liquid crystal display device is driven by positively and negatively inverting the applied display signal so as to correspond to each field.
Further, as disclosed in Japanese Patent Publication No. 55-6916, there is a known technique for driving a liquid crystal display device with an alternating current having symmetrically positive and negative polarity by inverting the polarity of an electric signal to be applied to the liquid crystal so that direct current components of the signal are not included in the applied signal.
As mentioned above, when the TFT color liquid crystal display device is driven with an alternating current by positively and negatively inverting the display signal, corresponding to each field, a signal sent from a signal line is inverted and the inverted signal is inputted to pixels. Conventionally, there are several methods for inverting the signal sent from the signal line. The simplest method is xe2x80x9cinversion-to-fieldxe2x80x9d in which a signal sent from a signal line is inverted in units of fields, corresponding to pixels.
Other methods which are employed are xe2x80x9cinversion-to-gate-linexe2x80x9d in which a cycle of the inversion corresponds to each scanning line, and xe2x80x9cinversion-to-data-linexe2x80x9d in which a cycle of the inversion corresponds to each signal line. In addition, in order to eliminate cross-talk and insufficient writing more efficiently than these two methods, xe2x80x9cdriving-by-inversion-to-dotxe2x80x9d in which a signal is inverted corresponding to adjacent dots is also employed.
By referring to FIGS. 7 and 8, both the occurrence mechanism of a sticking phenomenon and the occurrence mechanism of flickers in a liquid crystal device will be described below.
FIG. 7A shows the most general schematic structure of a liquid crystal cell that constitutes a liquid crystal display device. A condition in which liquid crystal 1 is encapsulated between a pair of transparent substrates is shown. On both electrodes with respect to the liquid crystal 1, electrode layers 2 and 3, insulating films 4 containing SiNx thereon, and orientation films 5 containing polyimide thereon are generally formed. FIG. 7A also shows a condition in which a voltage is not applied to the liquid crystal cell, and ions which are inevitably present in the liquid crystal 1 are dispersed at random.
When a direct current voltage is applied to the liquid crystal cell as shown in FIG. 7B, this voltage causes the ions present in the liquid crystal 1 to be polarized and absorbed to the surfaces of the orientation films 5. In other words, since the SiNx-contained insulating films 4 and the polyimide-contained orientation films 5 (both are insulating films) are present on the electrode layers 2 and 3 in the liquid crystal cell, which is conventional, the ions are absorbed to form capacitors CSINxxc2x7PI between the surfaces of the orientation films 5 and the surfaces of the electrode layers 2 and 3.
When the capacitors CSINxxc2x7PI is formed, an asymmetric voltage VAS (voltage generated by sticking) is generated by the absorbed ions. As shown in FIG. 7C, in the liquid crystal cell a relationship expressed as VAS=Qion/CSINxxc2x7PI is established, where Qion represents the charges of the absorbed ions.
In a condition in which the above-described sticking occurs and the absorbed ions are fixed, as shown in FIG. 7C, when the voltage between the electrode layers 2 and 3 is expressed as V0, a relationship expressed as V0=VAS1 (asymmetric voltage to the electrode layer 2)+VAS2 (asymmetric voltage to the electrode layer 3)+VLc (actually applied voltage to the liquid crystal 1) is established. If an applied voltage from the exterior is set to zero with this condition, a voltage expressed as VLC=VAS1+VAS2=VAS is applied to the liquid crystal cell as shown in FIG. 7D. This is the result of the sticking, which affects display quality, as a residual image in display. Further, if the liquid crystal cell is driven with an alternating current, the voltage of VAS is a factor in the occurrence of flickers.
The occurrence mechanism of flickers will be described below.
To avoid sticking caused by driving the liquid crystal with a direct current as described above, the liquid crystal is driven with an alternating current as shown in FIG. 8A. In a condition in which the liquid crystal is driven with an alternating current while the asymmetric voltage (VAS) is being generated, even when the voltage xc2x1V0 is applied, in connection with the signal applied to one pixel as shown in FIG. 8B, the positive polarity has a relationship expressed as V0xe2x88x92VAS=|VLC| and the negative polarity has a relationship expressed as V0+VAS=|VLC|, so that the applied voltage has different magnitude in polarity.
When the electro-optic characteristics of the liquid crystal is as shown in FIG. 8C, in other words, when the relationship of transmittance (T) with respect to applied voltage (V) is represented by a curve shown in FIG. 8C, transmittance of T(V0) must be obtained with respect to the applied voltage (V0). However, this case has asymmetric voltage VAS, the transmittance differs depending upon the polarity. The transmittance in the positive and negative polarities are expressed as follows:
transmittance in positive polarity T(V0xe2x88x92VAS);
transmittance in negative polarity T(V0+VAS).
Consequently, flickers occur, and the amplitude of a change in the transmittance is expressed as the following relationship: xcex94T=T(V0xe2x88x92VAS)xe2x88x92T(V0+VAS)
Based on the described background, the liquid crystal is driven by inverting the signal. Such driving by inversion needs voltage amplitude which is twice as much as necessary voltage amplitude for normally driving the liquid crystal, resulting in large power consumption. For example, if a voltage necessary for driving the liquid crystal is 5 V, a signal needs to have 10 V (namely, xc2x15 V) in positive and negative polarity.
In addition, power consumption P caused by such a driving signal is generated by charging or discharging the liquid crystal cell serving as a capacitor at the inverting frequency of the signal. Thus, the following relationship is established:
Pxe2x88x9df, V2
where P represents power consumption; f, the inverting frequency of the signal; and V, voltage. As expressed in the above relationship, P is proportional to f, and P is proportional to V2.
Therefore, power consumption necessary for applying the source voltage (Vsig) when the polarity of the signal is inverted needs to be not less than four times greater than power consumption when the polarity is not inverted, so that a problem of large power consumption occurs. Further, if the inverting frequency is considered, the difference between the two cases becomes larger.
Accordingly, in view of the foregoing problems, it is an object of the present invention to provide a liquid crystal display device and a driving method therefor, which device can reduce power consumption, have increased numerical aperture, correct a change in its electro-optic characteristics caused by a sticking phenomenon, and realize gradation display, contrast and flickering as designed.
In accordance with an aspect of the present invention, the foregoing object is achieved through the provision of a liquid crystal display device having a pair of substrates, one of which is provided with a thin-film-transistor circuit, the other one of which is provided with a common electrode, with liquid crystal provided therebetween, the thin-film-transistor circuit including thin-film-transistors and pixel electrodes in regions surrounded by both gate lines and source lines arranged in the form of a matrix, wherein, when the liquid crystal display device is driven, the polarity of an electric signal to be applied to the liquid crystal is fixed corresponding to a plurality of frames or fields, or all frames or fields.
Preferably, an offset voltage whose polarity is the same as the polarity of an asymmetric voltage generated by sticking in the liquid crystal is included in the signal for driving.
Preferably, the liquid crystal cell of the liquid crystal display device is forced to have sticking, and an offset voltage whose polarity is the same as the polarity of an asymmetric voltage generated by the sticking is included in the signal for driving.
In accordance with another aspect of the present invention, the foregoing object is achieved through the provision of A driving method for a liquid crystal display device having a pair of substrates, one of which is provided with a thin-film-transistor circuit, the other one of which is provided with a common electrode, with liquid crystal provided therebetween, the thin-film-transistor circuit including thin-film-transistors and pixel electrodes in regions surrounded by both gate lines and source lines arranged in the form of a matrix, wherein the driving method includes the step of fixing the polarity of an electric signal to be applied to the liquid crystal so as to correspond to a plurality of frames or fields, or all frames or fields.
Preferably, the driving method includes the step of including an offset voltage whose polarity is the same as the polarity of an asymmetric voltage generated by sticking in the liquid crystal in the signal for driving.
Preferably, the driving method includes the steps of: forcing the liquid crystal cell of the liquid crystal display device to have sticking; and including an offset voltage whose polarity is the same as the polarity of an asymmetric voltage generated by the sticking in the signal for driving.
As described above, in accordance with the present invention, the polarity of an electric signal to be applied to liquid crystal is fixed corresponding to a plurality of frames or fields when the liquid crystal device is driven. Thus, power to be applied across the electrodes is reduced much less than that in the conventional liquid crystal device.
In the conventional structure or driving method that utilizes xe2x80x9cdriving-by-inversion-to-linexe2x80x9d in which a liquid crystal device is driven by oppositely inverting the polarity of a signal so as to correspond to each line, xe2x80x9cdriving-by-inversion-to-dotxe2x80x9d in which a liquid crystal device is driven by inverting the polarity of a signal so as to correspond to each dot, a difference in voltages applied to pixel electrodes both adjacent to the gate electrode is large, its polarity being inverted, and as a result, there is considerable irregularity in the direction of an electric field in proximity to the gate electrode, and in this portion irregularity readily occurs in the orientation of liquid crystal. However, in accordance with the structure or driving method of the present invention, a difference in voltages applied to pixel electrodes both adjacent to the gate electrode is small, the polarity of these voltages being almost not inverted, and as a result, there is small irregularity in the direction of the electric field in proximity to the gate electrode, and in this portion irregularity hardly occurs in the orientation of liquid crystal. In particular, when the polarity of the signal is not inverted at all, an advantage in which inversion of the polarity of the electric field does not occur in the whole liquid crystal cell is obtained.
Consequently, the conventional structure has regions covered with a black mask for the orientation irregularity to hardly be seen, while the present invention provides regions without being covered are expanded, which advantageously increases a numerical aperture for a liquid crystal device. Accordingly, by employing the structure according to the present invention, the brightness of a liquid crystal display device can be enhanced when the power consumption of its backlight is the same as that in the conventional liquid crystal display device, in addition, when its brightness is similar to conventional, power consumption resulting from reduced power consumption in its backlight can be reduced.
In addition, when an asymmetric voltage is generated by sticking in liquid crystal, by including an offset voltage whose polarity is the same as the polarity of an asymmetric voltage generated by sticking to the liquid crystal in the signal for driving, the liquid crystal cannot be affected by this asymmetric voltage, and flickers can be suppressed. Accordingly, gradation display, contrast and flickers are advantageously realized as designed.